These legendary giants were among the children of Asag that the Lords of Men drove into the Northern Wastes. The consequences of the war of men against the remnants of Asag's brood pepper the Fields of Woe, where it's believed the corpses of the Colossi remain petrified in the poses of their final transfiguration.
Scholars in Mamara believe the Colossi were once a race of people, like the Yevim or the Rasa that roamed the Midworld before the Yevim took dominion in the skies. Whether they were giant golems or flesh-and-blood creatures like the other children of Asag is unknown, but the pitiable statues that are purported to represent their mournful poses after whatever apocalyptic harm befell them in the Field of Woe suggest they had lifelike sentience all their own.